Glycogen storage disease type II (GSDII) results from deficiency of acid a-glucosidase (GAA), a lysosomal enzyme that degrades glycogen. GSDII causes cardiorespiratory failure in infants, and progressive respiratory failure in juveniles and adults. Although respiratory failure has been attributed to muscle pathology, autopsy case reports show cervical spinal glycogen accumulation and suggest motoneuron pathology. We have observed that efferent phrenic discharge, mean inspiratory airflow, and the ratio of minute ventilation to metabolic rate (VE/VCO2) are blunted in a murine GSDII model, the GAA-/- "knockout" mouse. In addition, a selective knockout mouse with normal skeletal muscle contractility and GAA expression, but no GAA in the central nervous system (CNS), also exhibits marked reductions in phrenic output and ventilation. Accordingly, CNS GAA deficiency is associated with impaired respiratory motor output, and respiratory insufficiency in GSDII may reflect both a neural and a muscular pathology. A neural mechanism is also implicated by preliminary data showing extensive cervical spinal glycogen accumulation in GAA-/- mice, particularly within retrogradely identified phrenic motoneurons. Determining the mechanisms underlying respiratory insufficiency is important because i.v. enzyme replacement (the current clinical GSDII therapy) does not target the CNS as GAA cannot cross the blood brain barrier. A promising method for targeting both muscle and the CNS is recombinant adeno-associated virus (rAAV) gene therapy. Our preliminary data indicate that rAAV effectively transfects phrenic motoneurons, and can be delivered by intraspinal or intrathoracic injection. Further, ventilation is significantly enhanced in GAA-/- mice one month following intrathoracic injection of rAAV packaged with the GAA gene (rAAV-GAA therapy). These experiments represent a unique collaboration between laboratories specializing in respiratory physiology (Fuller), gene therapy (Byrne), and neuroanatomy (Reier). We propose to test three hypotheses: 1) neural drive to the diaphragm and ventilation are attenuated in both the selective and full GAA-/- knockout mice;2) respiratory deficits in these mice occur in parallel with glycogen accumulation in phrenic motoneurons, and 3) intraspinal and intrathoracic rAAV-GAA delivery can ameliorate spinal glycogen accumulation and enhance respiratory motor output in the selective and GAA-/- knockout mice, respectively.